The present invention relates to the transfer of electromagnetic energy between a transmission coil and a receiver coil. More particularly, the invention relates to a pouch, secured to the skin of a patient, in which a transmission coil may be temporarily placed or held while electromagnetic energy is transferred to an implantable device through the transmission coil. Such electromagnetic energy transfer thereby allows for the transmission of power and/or data to the device and/or to a replenishable power source, e.g., a rechargeable battery, associated with the implantable device.
Various types of medical devices such as cochlear implants, neural muscular stimulators, implantable pumps, and other implantable devices have been developed that are designed to be surgically inserted within a patient's body to carry out a medically related function for an extended period of time. Although a lead connected to the implanted device and extending outside the patient's body can be used to supply electrical power required to energize the device and/or control data, any lead that passes through the skin increases the risk of infection if left in place for more than a few days.
As an alternative to having a lead or wire pass through the skin of the patient, power and/or data can be supplied to an implanted medical device via an RF or electromagnetic link that couples power from an external (non-implanted) coil to an internal (implanted) coil. So long as a suitable link, e.g., an inductive link, is established between these two coils, which means some sort of external power source must be carried by or worn by the patient, power and/or data can be continuously supplied to the implanted medical device from the worn or carried external device, thereby allowing the implanted medical device to perform its intended function.
It is also known to power an implanted medical device with a battery that is housed internal to the implanted device. However, any battery used for extended periods of time will eventually need to be either recharged or replaced. Replacing an internally implanted battery subjects the patient to further surgery and thus is not desirable, at least not on a frequent basis.
Rather than replace an implanted battery, the battery can be recharged by transcutaneously coupling power from an external source to an implanted receiver that is connected to the battery. Although power can be coupled from an external source at radio frequencies using matching antennas, it is generally more efficient to employ an external transmission coil and an internal receiving coil which are inductively (electromagnetically) coupled to each other to transfer power at lower frequencies. In this approach, the external transmission coil is energized with alternating current (AC), producing a varying magnetic flux that passes through the patient's skin and induces a corresponding AC voltage in the internal receiving coil. The voltage induced in the receiving coil may then be rectified and used to power the implanted device and/or to charge a battery or other charge storage device (e.g., an ultracapacitor), which in turn powers the implanted device. For example, U.S. Pat. No. 4,082,097 discloses a system for charging a rechargeable battery in an implanted human tissue stimulator by means on an external power source.
Some implantable devices, such as neural or auditory stimulators, do not require internal batteries as a power source, but rather receive power directly via a transcutaneous coupling. Still other implantable devices, in addition to receiving power directly from an external power source, may also transmit information and data back to an external device relating to the status of the device and the signals it senses in the patient's body. See, e.g., U.S. Pat. No. 5,603,726, which describes an implantable cochlear stimulator powered by an external wearable system; and U.S. Pat. Nos. 5,324,316; 5,312,439; and 5,358,514; which describe a small implantable microstimulator. All of these patents--the '726 patent, the '316 patent, the '439 patent, and the '514 patent--are incorporated herein by reference.
When electromagnetic coupling is used to transfer power and/or data to an implanted device, proper alignment of the external device and the implanted device is important for effective electromagnetic coupling. A common way of achieving the desired alignment between the external transmission coil and the implanted receiver coil is to employ a permanent magnet in both the headpiece which houses the external coil and the implanted device which houses the receiver coil. The magnetic attractive force associated with such magnets holds the external coil in close proximity to the receiver coil and provides the desired alignment between the coils so that inductive coupling may efficiently occur.
Another method of aligning an external unit with an implanted internal receiving device is shown in U.S. Pat. No. 5,545,191. In this patent, the external unit uses VELCRO.RTM. strips for attaching the external unit to the skin in a proper location for optimal electromagnetic coupling between the units.
As is known in the art, the efficiency with which electromagnetic power may be transcutaneously transferred between a transmission coil and a receiving coil, where one of the coils is implanted and the other is not, is a function of the alignment and distance between the coils. It is thus desirable to position the external device as close as possible to the implanted device.
Disadvantageously, existing external devices that supply electromagnetic power are bulky and large. These devices include a power source, control circuitry and transmission coil. The power source (e.g., a battery and control circuitry) is usually attached to a person's belt or pocket. The transmission coil must be placed on the skin at a location that is as close as possible to the implanted device. Such positioning has heretofore required some sort of alignment mechanism, e.g., permanent magnets in the implanted device and in an external head that carries the transmission coil. The use of such permanent magnets may not be desirable in all situations, and adds to the cost of the system. The transmission coil must also be attached to the power source and control circuitry via some sort of cable, which cable (depending upon how often the transmission coil must be used) may be a nuisance to the patient, and something that the patient considers obtrusive and unsightly.
In view of the above, it is evident that what is needed is a convenient way to easily align the external transmission coil, when used, with the implanted device, without the need to use permanent magnets. What is further needed is an unobtrusive external device that can transmit power and/or data transcutaneously to an implanted device, and wherein such external device is not only small and light weight, but is also readily attachable to the skin in close proximity to, i.e., substantially aligned with, the implanted device.